Currently, subcutaneous implantable medical devices (SIMD) are provided for a variety of cardiac applications. The SIMD include a “housing” or “can” and one or more electrically-conductive leads that connect to the can through an electro-mechanical connection. The can is implanted outside of the heart, in the pectoral region of a patient and contains electronics (e.g., a power source, microprocessor, capacitors, etc.) that provide pacemaker, cardioversion and/or defibrillation functionality.
Recently, small sized devices have been proposed for intra-cardiac implant within the heart. These devices, termed leadless pacemakers or leadless implantable medical devices (LIMDs), are typically characterized by the following features: they are devoid of leads that pass out of the heart to another component, such as a pacemaker can outside of the heart; they include electrodes that are affixed directly to the can of the device; the entire device is attached to the heart; and the device is capable of pacing and sensing in the chamber of the heart where it is implanted.
LIMDs that have been proposed thus far offer limited functional capability. These LIMDs are able to sense in one chamber and deliver pacing pulses in that same chamber, and thus offer single chamber functionality. For example, an LIMD that is located in the right atrium would be limited to offering AAI mode functionality. An AAI mode LIMD can only sense in the right atrium, pace in the right atrium and inhibit pacing function when an intrinsic event is detected in the right atrium within a preset time limit. Similarly, an LIMD that is located in the right ventricle would be limited to offering VVI mode functionality.
Various limitations and tradeoffs exist when deciding whether to implant an SIMD or an LIMD. An LIMD represents a new leadless technology that may afford an opportunity to mitigate risks associated with lead failures and replacements. A leadless SIMD has also been created for the same lead issues and cosmetic reasons.
Transvenous pacemakers provide single, dual chamber and CRT/BiV pacing modalities while first generation LIMDs are single chamber right ventricular only devices. Further, first generation S-ICDs have been reported to be prone to T-wave over-sensing especially in the younger and pediatric patient groups. It has been shown that atrial-ventricular (AV) synchronous LV pacing may be desirable over RV pacing alone. In many cases, LV pacing provides the same benefits as cardiac resynchronization therapy CRT bi-ventricular pacing.
A need remains for a unified system that delivers AV synchronous pacing to the left ventricle as well as high voltage therapy.